Improvements in digital signal processing of terrain data, particularly data compression, storage, retrieval and reconstruction, have resulted in the development of dynamic display mechanisms which provide an aircraft pilot/observer with a simulated display (e.g. moving map or radar) of terrain over which an aircraft is flying or is simulated to be flying in accordance with stored digital data and flight path inputs from the aircraft's navigation system. For example, in copending U.S. patent application Ser. No. 168,437 filed March 15, 1988, which is a continuation of Ser. No. 641,179, filed Aug. 15, 1986, now abandoned which is a continuation of application of Ser. No. 224,742 filed January 13, 1981, now abandoned, by Paul B. Beckwith Jr. et al, entitled "Digital Map Generator and Display System", and assigned to the assignee of the present application, there is described a system for effecting the dynamic display of terrain data, which is compressed and stored in digital form and which may be selectively and controllably accessed from memory and viewed on a cockpit cathode ray tube display in the form of a moving map of the terrain over which the aircraft is flying (or is simulated to be flying). Similarly, in copending U.S. patent application Ser. No. 652,875, filed Sept. 21, 1984, by Paul B. Beckwith Jr. et al, entitled "Digital Radar Generator", now U.S. Pat. No. 4,702,698, issued October 27, 1987 also assigned to the assignee of the present application, there is described a video display system for generating a simulated radar image corresponding to a prescribed radar sweep pattern directed toward the terrain from a designated radar source reference point about a preselected look angle. Like the moving map system, the digital radar display simulator processes stored digital data representative of elevations of terrain over a grid or matrix of geographical map entries.
In systems of the type described in the above-identified applications, because of size, weight and cost constraints, the capacity of the elevation data terrain map storage medium (e.g. magnetic disk), is limited, thereby restricting the resolution of the image to be produced from such a data base, without the use of a mechanism that effectively decreases the spacing between data points from that in the original data base (typically on the order of 100 meter intervals as provided by the Defense Map Agency). This limited capacity of the terrain map storage medium can be tolerated by using either some form of compression/decompression algorithm (e.g. discrete cosine transform) through which the data is subjected to upstream processing prior to storage and then again upon retrieval from the storage medium, or an interpolation mechanism through which additional elevation data values for map locations between known data values map be derived and thereby provide an enhanced, cockpit quality image.
Now, although a compression/decompression scheme permits the storage of an increased volume of data within a given data base capacity, its use adds to system complexity and cost prior to the signal processing that is to be carried out by the image processor. In addition, a particular compression/decompression scheme in and of itself may or may not increase resolution; if not, there still needs to be a mechanism for estimating data values between known locations. In the past, interpolation of elevation data values for points lying between successive known data values of a stored terrain map has been accomplished through an averaging or linear estimation process, typically based upon the data values of the successive known points. Examples of such interpolation schemes include linear or straight line connecting of successive elevation magnitudes and proportionately estimating data values of intermediate points, and through the use of a polynomial whose coefficients are derived using an arithmetic or geometric mean. For high precision imagery processing, such as high resolution radar returns, the above-referenced conventional signal processing methodologies have been found to be unsatisfactory; they either corrupt the original data, or they produce estimated data values that introduce imagery effects that would not otherwise appear in the contour of the original terrain.
More particularly, high precision imagery processing of (simulated) radar return data requires that both the elevation be maintained and the slope of the interpolated terrain be continuous at the original data points. In addition, the introduction of ripple or over(under)shoot into the interpolated contour between data points cannot be tolerated, since it would produce unreal shadows in the processed image. In other words, the contour of the interpolated radar image must be smooth and without a change in sign of the slope of the terrain between original data points.